Not Applicable
Not Applicable
1. Field of Invention
This invention relates to an apparatus and method for treating wastewater, specifically to a biological reactor and biological process for treating wastewater comprising organic pollutants and/or wastewater nutrients under aerobic, anoxic, or anaerobic conditions.
2. Description of Prior Art
The wastewater containing organic pollutants is usually treated using a biological process. The suspended-growth process, which is also known as the activated sludge process, is one of the most widely used biological processes. For example, most municipal wastewater treatment plants employ the activated sludge process in their secondary treatment stage for removing organic pollutants from the wastewater. The conventional activated sludge process comprises a suspended-growth bioreactor (conventionally referred as the aeration tank when operated in aerobic conditions) and a clarifier (conventionally referred as the secondary clarifier). The wastewater and the return activated sludge from the clarifier flow into the aeration tank. Air or oxygen is supplied to the aeration tank through an aeration system. In the aeration tank, pollutants are either degraded or adsorbed by the activated sludge. The aeration tank mixed liquor then enters the clarifier for solid-liquid separation. The supernatant of the clarifier is discharged through the clarifier outlet. Most of the settled sludge in the clarifier is returned back to the aeration tank. Excess sludge is wasted to a sludge handling system for further treatment.
In most cases, the wastewater also contains organic nitrogen, ammonia, and phosphorus. They are called wastewater nutrients because they can cause the excessive growth of algae in the receiving water body. In addition, the organic nitrogen and ammonia consume oxygen in the receiving water body during their oxidation. These wastewater nutrients can also be removed in the bioreactor. Microorganisms can convert organic nitrogen and ammonia to nitrate or nitrite under aerobic conditions. This process is called nitrification. If the bioreactor is under anoxic or anaerobic conditions (no dissolved oxygen (DO) presents), microorganisms can reduce the nitrate and nitrite to nitrogen gas. This process is called de-nitrification. If the bioreactor is maintained in low DO aerobic conditions, simultaneous nitrification/de-nitrification can be achieved. If the aerobic sludge continuously passes through an anaerobic or anoxic zone in the bioreactor, a group of microorganisms favorable for phosphorus uptake can be acclimated.
Microorganisms are work force for pollutant removal. The performance of the bioreactor can be enhanced if the activated sludge concentration is increased.
In conventional continuous-flow suspended-growth bioreactors, the microorganism population is maintained through the return of the concentrated activated sludge from the clarifier. Thus, the microorganism concentration in the bioreactor is dependent on the suspended solids concentration in the return activated sludge and the sludge return rate.
The clarifier is designed to clarify the bioreactor outflow using gravity settling. The performance of the clarifier is dependent on the hydraulic loading and total solids mass entering the clarifier. Increasing the sludge return rate may initially increase the microorganism concentration in the bioreactor. However, this may increase the hydraulic and solids loadings to the clarifier because of the increased effluent flow rate from the bioreactor and the increased sludge concentration in the effluent. Therefore, the clarifier performance could be reduced, resulting in the increased solids concentration in the clarifier effluent, decreased solids concentration in the return activated sludge, and the increased sludge blanket level in the clarifier. This could cause the system failure.
Objects and Advantages
Accordingly, the objects and advantages of my invention are:
(a) to provide a bioreactor that has an internal sludge return function to supplement the conventional sludge return practice to maintain a higher microorganism concentration in the bioreactor and improve the bioreactor performance;
(b) to reduce the sludge return rate from the clarifier to simplify the operation and save energy;
(c) to reduce the total hydraulic loading and solids loading to the clarifier to improve the clarifier performance, i.e., improve the clarifier effluent quality and increase the solids concentration in the clarifier underflow.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.
The present invention is a suspended-growth bioreactor that has an internal sludge return function to supplement the sludge return from the clarifier for treating wastewater. Said bioreactor comprises a mixing zone for pollutant degradation, a static zone for sludge settling and thickening, and a mixed liquor circulation channel that provides a continuous circulation flow for carrying the settled sludge from the static zone to the mixing zone. Thus, the bioreactor of this invention can maintain a higher microorganism concentration than conventional suspended-growth bioreactors, resulting in improved wastewater treatment performance.
The bioreactor of this invention can be maintained in normal aerobic conditions to enhance organic matter removal and/or nitrification. It can also be maintained in low DO aerobic conditions for simultaneous organic matter degradation, nitrification, and de-nitrification. It can also be maintained in anoxic conditions for de-nitrification. Moreover, it can be maintained in anaerobic conditions for anaerobic degradation of organic pollutants. More than one bioreactor of this invention, which may be operated in different conditions, can be connected in series for wastewater treatment.